The present invention relates to an improved tread contour for radial ply passenger vehicle tires and more specifically to the tread contour of types of radial passenger vehicle tires that include conventional tires, tires capable of being used in the uninflated condition, and tires constructed with plies reinforced with cords having a modulus of greater than 100 GPa.
Treads designed specifically for the passenger vehicles have been designed such that the cross-sectional contour of the tread has been defined according to multiple radii of curvature which are joined in such a way that adjacent curves. meet one another tangentially, i.e., without discontinuity in the axially directed curvature of the tread. With this method, to fit a required drop to a tread contour (i.e., drop as measured from the centerline to the shoulder of the tire) with one or more radii, the tread contour shape required one or more curves with low radius of curvature. The result of this design criteria was a footprint shape factor that provided a satisfactory pressure distribution across the footprint during both straight and turning driving patterns.
In the particular case of runflat type tires having generally thicker sidewalls than conventional tires, if the tread shoulder contours are described by tangentially joined adjacent curves having smaller radii of curvature in the regions of the tread shoulders than the radii of curvature in the central tread portion, bending stresses tend to be transmitted from the sidewalls to the tread during conditions of low to zero air pressure in the tire. The effect of this bending stress between the sidewalls and the tread is to cause inward buckling of center portion of the tread, the result of which is that the center portion of the tread receives little to none of the tire""s load when the tire""s pressure is low to zero. One result of this inward buckling of the tread during runflat conditions is the potential for reduced vehicle handling characteristics in straight-run operation and, especially, in cornering. Another result is shorter operational life of the tread in the runflat condition, due to the cycling of severe inward bending stresses in the tread center. Yet another result associated with tire operation in the runflat condition is the tendency for the shoulders of the tire to come into contact with the road, causing yet further reduction of optimal handling in straight line operation and especially in cornering.
The adverse effects of tread contours defined by tangentially adjacent circular curves described in the paragraph above in relation to runflat tires apply generally to all tires that have tread contours which are defined by tangentially adjacent circular curves.
European Patent Application EP-A-0 424 155, (Sumitomo Rubber Industries Limited, filed Oct. 18, 1990, discloses a pneumatic radial tyre in which the tread surface is formed along a curvature plane which includes a first arc with a radius R1, a second arc with a radius R2, a third arc with a radius R3, and a main circumferential groove.
The development of tires with a high modulus carcass, containing, for example, wire cords in the ply, leads to tire treads that tend not to flatten in a way that optimally distributes the loading on the ground; more specifically, a high modulus carcass having low radii of curvature tread contour shapes in the shoulder region, as conventionally desired to achieve the required drop, tends to cause the tire""s load to be disproportionately borne by the shoulders and sides of the tread. This leads to less than optimal footprint pressure distribution in straight driving and to low footprint contact surface in cornering. More specifically, conventional design criterion can result in less than optimal wear characteristics, and less than optimal handling and ride characteristics that correspond to a footprint shape factor in which excessive portions of the tire""s load are carried by the outer-most portions of the tread. It would be desirable to provide a tread contour shape being more evenly distributed across the entire tread width in such a way as to yield a satisfactory footprint shape factor.
An object of the present invention is to provide a low-aspect ratio passenger-vehicle tire having a tread design that gives long tire life, good handling and satisfactory ride as a result of a tread design having decoupling grooves which define a circumferential line at the bottom thereof which is a region of great flexibility to decouple sidewall bending stresses from the central ribs of the tire.
Another object of the present invention is to provide circumferential decoupling grooves at the meeting point of adjacent contour-defining curves of ribs that cannot contain a single line which is mutually tangent to the contour defining curves so as to enable a large area of contact between the tire tread and the road on runflat radial tires.
Still another object of the present invention is to enable a large area of contact between the tire tread and the road on radial tires in which the radial plies include high modulus reinforcing cords and the tread incorporates the decoupling grooves of the present invention.
Yet another object of the invention is to enable a large area of contact between the tire tread and the road on radial tires having more or less conventional carcass design and construction.
The present invention relates to a pneumatic radial ply passenger tire having a tread, a casing with two sidewalls, one or more radial plies extending from and wrapped about two annular beads and a belt reinforcement structure located radially between the tread and the plies. The tread is characterized by the tread having a central rib with a first contour-defining curve extending outwards from the mid centerline of the tire towards the sidewalls and a pair of side ribs each having a second contour-defining curve disposed between the central rib and the sidewalls. The first and second contour-defining curves each have first and second radii disposed such that the meeting point of first and second adjacent contour-defining curves cannot contain a single line that is mutually tangential to the first and second adjacent contour-defining curves. First and second circumferential decoupling grooves are disposed on either side of the mid centerline at the meeting point-of the first and second adjacent contour-defining curves. The first and second circumferential decoupling grooves, which are disposed in a bilaterally symmetrical location on either side of the mid centerline, define a circumferential line at the bottom of the first and second grooves which is a region of greater flexibility than the adjacent ribs of the tire tread to decouple sidewall bending stresses from the central rib. Further, the first and second circumferential decoupling grooves have parallel, zig-zag or sinusoid shaped, spaced side walls.
In one embodiment of the invention, the pneumatic radial ply passenger tire has sidewalls with at least one ply reinforced with cords having a modulus E of X, X being at least 10 GPa and a second ply reinforced with substantially inextensible cords having a modulus E greater than X of the cords of the other ply. In this embodiment, an insert disposed between the first and second plies in the sidewalls results in a runflat type tire. This tire can also include a second pair of ribs between the second pair of ribs and the sidewalls. Each of the second pair of ribs has a third contour-defining curve disposed between the first pair of ribs and the sidewalls. Third and fourth circumferential decoupling grooves are disposed between the second and third adjacent contour-defining curves to help decouple the sidewall stresses from the central and side ribs.
In still another embodiment, the pneumatic radial ply passenger tire has sidewalls having at least one ply reinforced with cords having a modulus of over about 100 GPa. The tire of this embodiment also incorporates the decoupling groove in accordance with the invention.